The increased demand in the field of power conversion—size reduction and high-efficiency is difficult to meet with the present available power converters. Some features inherent in devices of relevant technology militate against achievement of the desired goals. Some converters are specifically designed to handle a wide range of output loads and variations of input voltage, resulting in less efficiency. Reducing the size of components (capacitors and magnetic component) related to high switching frequency and as results to concomitant high switching losses.
Common approach of ZVS and ZCS topology disclosed in the prior art is series resonant converter described in e.g. USOORE33866E, U.S. Pat. No. 6,178,099B1 that achieves nearly lossless soft switching using pulse width modulation and frequency super-resonant control of commutation in full or half bridge converter. Soft switching converters attempt to take advantage of the parasitic effect of components within the converter in order to reduce the voltage potentials across (and current flows through) the switches before effecting a switching operation. More specifically, soft switching converters adjust the switching timing in order to charge and to discharge the parasitic switch capacitances of the transistors through the use of current supplied by the magnetizing inductance of the winding of the transformer, thereby reducing the voltage across Off or Open transistors, and current flow through On or Closed transistors. During the time switching which transistors takes, the soft switching reduces the power losses during the switching operation, thereby enabling the converter to operate at high frequency, high efficiency and with reduced electromagnetic interference.
However, the operation with typical pulse width modulation or the variation of switching frequency is a deficient in full volume of line and maximal load alternation which leads to stresses on the switching devices which are similar to those of a hard switched converter.
At the series resonant converters disclosed in, e.g., U.S. Pat. No. 4,855,888 and EP 0503862B1 there is presented the topology of two-stage (full or half bridge) circuitry connected to input voltage. In the similar devices, such as those disclosed in U.S. Pat. Nos. 5,999,417 or 6,930,893 two stages are floating relatively to one another, where the output voltages are algebraic sum voltages of the output voltages of the two stages. The regulation of the output voltages is achieved by alteration of the phase between the two stages of the converter. However, the operation in full volume (from 0 to max) of loads and line variation leads to inevitable hard switching, e.g., excessive switching losses, excessive EMI and the like.
Quasi-resonant bidirectional converters were introduced in U.S. Pat. No. 4,663,699 and Natural ZVS Medium-Power Bidirectional DC-DC Converters with Minimum Numbers of Devices, Hui Li Fang Zheng Peng, J. S. Lawlee, IEEE, Vol. 39, No. 2, March/April 2003. Presented two stage active half-bridge topology featured ZVS in full volume of line and load variation. However, high current switching leads to high level conductive and switching losses, and causes the limitation to power, obstacles to decrease the cost, volume and EMI.
From this prior art other challenge is presented to minimize losses during the switch transition time of two stage active full bridge or half-bridge converter.